Valley spin hall effect based non-volatile memory

ABSTRACT

A memory cell is disclosed which includes a semiconductor layer, a first electrode coupled to the semiconductor layer, a second electrode coupled to the semiconductor layer, wherein the first and second electrodes are separated from one another along a first axis and wherein the semiconductor layer extends beyond the first axis along a second axis substantially perpendicular to the first axis, thereby forming a first wing, a third electrode separated from the semiconductor layer by an insulating layer, a first magnetic tunnel junction (MTJ) disposed on the first wing, and a first read electrode coupled to the first MTJ.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application a continuation application of U.S.Non-Provisional application Ser. No. 16/909,971 filed Jun. 23, 2020, nowU.S. Pat. No. 11,250,896 to Thirumala et al., which is related to andclaims the priority benefit of U.S. Provisional Patent Application Ser.No. 62/865,280 filed Jun. 23, 2019, entitled “VALLEY SPIN HALL EFFECTBASED NON-VOLATILE MEMORY” the contents of each of which is herebyincorporated by reference in its entirety into the present disclosure.

STATEMENT REGARDING GOVERNMENT FUNDING

None.

TECHNICAL FIELD

The present disclosure generally relates to electronic memory, and inparticular, to non-volatile memories that operate based on valley spinhall effect.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

Over the last decade, there has been an immense interest in emergingnon-volatile memories (NVMs) due to their distinct advantages over thetraditional silicon-based memories, such as near-zero stand-by leakageand high integration densities. However, they possess design conflictsand issues associated with reliability, robustness, and high writeenergy. Spin-based memories using magnetic tunnel junctions (MTJs) lookpromising with good endurance and high integration densities.Specifically, spin-transfer-torque magnetic read access memory(STT-MRAM) has attracted immense interest. SAMSUNG's STT-MRAM in 28 nmFDSOI platform and INTEL's FinFET based MRAM technology are someindustrial efforts on the implementation of spintronic memory. However,there are several challenges which still need to be addressed. Forexample, these implementations exhibit low distinguishability betweentheir bi-stable states making them prone to sensing failures. Also, dueto their two-terminal cell-design, the write and read paths are coupled,leading to design challenges.

In addition, in the modern MTJ implementations utilizing giant spin hall(GSH) effect, the footprint for arrays of cells can be large since eachcell typically includes an access transistor. The access transistor canconstitute a large portion of the cell, thereby increasing the overallsize of the cell.

Therefore, there is an unmet need for a novel nonvolatile memory cellwith reduced footprint that is robust with respect to sensing failures.

SUMMARY

A memory cell is disclosed. The memory cell includes a conductive layer,an insulating layer disposed atop the conducting layer, a semiconductorlayer disposed atop the insulating layer, a first electrode coupled tothe semiconductor layer, and a second electrode coupled to thesemiconductor layer, The first and second electrodes are separated fromone another and wherein the semiconductor layer extends beyond the firstand second electrodes forming a first wing. The memory cell furtherincludes a third electrode coupled to the conductive layer. In addition,the memory cell includes a first magnetic tunnel junction (MTJ) disposedon the first wing. The first MTJ includes a first magnetic layer with afixed a polarity (pinned layer (PL)) in a first direction, a secondmagnetic layer (free layer (FL)) having a polarity that can be switchedfrom the first direction in which case the MTJ is in a parallel (P)configuration presenting an electrical resistance to current flow belowa resistance threshold to a second direction opposite the firstdirection in which case the MTJ is in an anti-parallel (AP)configuration presenting an electrical resistance to current flow higherthan the resistance threshold, and a non-magnetic layer (NML) separatingthe PL from the FL. the first MTJ presents an energy barrier (EB) toswitching from P to AP or AP to P. The memory cell further includes afirst read electrode coupled to the first MTJ.

An array made of memory cells is also disclosed. The array includes aplurality of memory cells dispersed in one or more rows and one or morecolumns. Each memory cell includes a conductive layer, an insulatinglayer disposed atop the conducting layer, a semiconductor layer disposedatop the insulating layer, a first electrode coupled to thesemiconductor layer, and a second electrode coupled to the semiconductorlayer. The first and second electrodes are separated from one anotherand wherein the semiconductor layer extends beyond the first and secondelectrodes forming a first wing. The memory cell also includes a thirdelectrode coupled to the conductive layer. In addition, the memory cellincludes a first magnetic tunnel junction (MTJ) disposed on the firstwing. The first MTJ includes a first magnetic layer with a fixed apolarity (pinned layer (PL)) in a first direction, a second magneticlayer (free layer (FL)) having a polarity that can be switched from thefirst direction in which case the MTJ is in a parallel (P) configurationpresenting an electrical resistance to current flow below a resistancethreshold to a second direction opposite the first direction in whichcase the MTJ is in an anti-parallel (AP) configuration presenting anelectrical resistance to current flow higher than the resistancethreshold, and a non-magnetic layer (NML) separating the PL from the FL.The first MTJ presents an energy barrier (EB) to switching from P to APor AP to P. THe memory cell also includes a first read electrode coupledto the first MTJ. Additionally the array includes a wordline (WL) foreach row of the one or more rows coupled to the third electrodes of eachof the associated memory cells in the associated row, a bit line (BL)for each column of the one or more columns coupled to the firstelectrodes of each of the associated memory cells in the associatedcolumn, a bit line bar (BLB) for each column of the one or more columnscoupled to the second electrodes of each of the associated memory cellsin the associated column, and a select line (SL) for each column of theone or more columns coupled to the first read electrodes of each of theassociated memory cells in the associated column. By selectivelyactivating the WL, BL, BLB, and SL, each of the memory cells is accessedfor writing or reading i) one memory cell at a time, ii) a row of memorycells at a time, iii) column of memory cells at a time, or iv) the arrayat a time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 a is a schematic of a typical magnetic tunnel junction cell.

FIG. 1 b is a schematic of a giant spin hall (GSH) effect in heavy metalleading to magnetization switching in an MTJ.

FIGS. 1 c and 1 d are a circuit schematics of GSH-MRAM which includes aread and write access transistor for single-ended memory (FIG. 1 c ),and a differential DGSH-MRAM (FIG. 1 d ) which shows a differentialDGSH-MRAM bit-cell schematic which includes a read and write accesstransistor for each side of the differential DGSH-MRAM cell.

FIGS. 2 a and 2 b are schematics of a single-ended memory device basedon valley spin hall effect magnetic read access memory (VSH-MRAM), FIG.2 a , and a differential memory device DVSH-MRAM, FIG. 2 b , accordingto the teachings of the present disclosure, each without an accesstransistor.

FIGS. 2 c and 2 d are cross-sectional representations of the VSH-MRAMand DVSH-MRAM according to the present disclosure.

FIGS. 2 e and 2 f are circuit diagrams for two sense amplifierconfiguration, single-ended in FIG. 2 e and differential in FIG. 2 f.

FIG. 3 a is a schematic of a band structure of WSe₂ showcasingspin-valley coupling resulting in VSH effect, according to the presentdisclosure.

FIGS. 3 b and 3 c are schematics of spin-transfer-torque (STT) forswitching perpendicular magnetic anisotropy (PMA) magnet and the novelidea of coupling VSH effect and spin torque for NVM design, respectivelyand according to the present disclosure), disclosing a valley-coupledspintronic devices with a small footprint without an access transistorfor energy-efficient data storage.

FIG. 4 a is a schematic of a four-probe measurement scheme used to makemeasurements for the devices of the present disclosure.

FIG. 4 b is a graph of resistance vs. V_(GS) showing the threeaforementioned types of resistance, i.e., ρ, R_(C), and R_(TOT) vs.V_(GS).

FIG. 5 a is a schematic of a non-local measurement setup for measuringparameters of the devices of the present disclosure.

FIGS. 5 b and 5 c are graphs of gate control of charge current (k) andNL resistance (R_(NL)=V_(NL)/I_(C)) for different device samples witharm lengths (L_(A)).

FIG. 5 d is a graph of R_(NL) (R_(NL)=V_(NL)/I_(C)) vs. arm length(L_(A)) of the memory device of the present disclosure.

FIG. 6 a is a flowchart for a self-consistent simulation framework forthe memory device of the present disclosure.

FIG. 6 b is a calibration graph of I_(GS) vs. V_(GS) for experimentalvs. simulation runs.

FIG. 6 c is a schematic of a distributed resistance network for sensingMTJs based on the conductance of WSe₂ layer and the shape of the readpath.

FIG. 6 d is a list of the simulations parameters used in the presentdisclosure.

FIG. 7 a are graphs of simulated gate voltage (V_(GS)) vs. modulatedcharge and spin current flow.

FIG. 7 b is a graph of magnetization vs. time for V_(DS)=1 V andV_(DS)=−1 V.

FIG. 7 c is a graph of switching time vs. V_(GS).

FIG. 8 a is a schematic of a memory array architecture for VSH-MRAM.

FIG. 8 b is a schematic of a memory array architecture for DVSH-MRAM.

FIG. 9 is a bar graph which illustrates the array level comparison ofthe memory designs of the present disclosure.

FIG. 10 is a bar graph of normalized memory energy for various SPECbenchmarks for DGSH, GSH, DVSH and VSH-MRAMs.

FIG. 11 is a schematic of a simulation framework, according to thepresent disclosure.

FIGS. 12 a and 12 b are bar graphs representing normalized system energyconsumption of VSH and GSH-MRAM for (a) synthetic and (b) realapplication benchmarks, respectively.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

In the present disclosure, the term “about” can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

In the present disclosure, the term “substantially” can allow for adegree of variability in a value or range, for example, within 90%,within 95%, or within 99% of a stated value or of a stated limit of arange.

A novel nonvolatile memory cell with reduced footprint that is robustwith respect to sensing failures is disclosed. This novel nonvolatilememory cell operates on the basis of valley spin hall (VSH) effect. Thenovel cell does not require an access transistor allowing a reduced sizefor the cell. In addition, the cell is designed to be robust withrespect to sensing failures by implementing a differential scheme.

Prior to discussing the specifics of the novel memory cell according tothe present disclosure, a discussion of technologies used in the memorycell of the present disclosure are provided including a discussion on atypical magnetic tunnel junction (MTJ) cell and giant spin hall (GSH)effect.

Magnetic Tunnel Junction (MTJ)

A typical MTJ cell is shown in FIG. 1 a . A typical spin-transfer-torquemagnetic read access memory (STT-MRAM) bit cell 10 is shown. TheSTT-MRAM bit cell 10 includes an access transistor 12 and a magnetictunnel junction (MTJ) stack 14. The MTJ Stack 14 is positioned between ahigh side node 24 and the access transistor 12 and includes aferromagnetic pinned layer 16 (which has a fixed magnetic orientation)and a free layer 18 (whose orientation can be switched), separated by atunneling oxide barrier 20. The access transistor is positioned betweenthe MTJ stack 14 and a low side node 26 and is controlled by a gate 22.The logic state stored in the bit cell depends on the relativeorientation between the free layer 18 and the pinned layer 16.Accordingly, for example parallel orientation (i.e., both layers havingthe same orientation) represents “0” and antiparallel orientationrepresents “1”. Two operations are of importance: a read operation and awrite operation. A read operation involves activating the gate 22 of theaccess transistor 12 and applying a bias voltage (Vread) between thehigh side node 24 and the low side node 26. A write operation isperformed by passing a current greater than a critical switching current(Ic) of the MTJ stack 14 (critical current is the current that can causethe free layer to switch from one orientation to another) for a minimumswitching duration. The current direction to write a “1” vs. a “0”differs based on the logic value to be written into the STT-MRAM bitcell 10. A read operation requires a small current (i.e., much smallerthan the critical switching current) to be passed through the STT-MRAMbit cell 10. This read operation can be accomplished based on avoltage-based sensing scheme wherein a small known current is applied tothe high side node 24 and resistance across the STT-MRAM bit cell 10 ismeasured. A higher resistance (e.g., between about 30 kohms and about 45kohms) represents a “1”—representing antiparallel orientation—while alower resistance (e.g., between about 10 kohms and about 15 kohms)represents a “0”—representing parallel orientation—where resistance ismeasured by comparing the voltage across the STT-MRAM bit cell 10against a reference voltage Vref (a voltage lower than the voltageassociated with the high resistance and higher than the voltageassociated with the low resistance); or based on a current-based sensingwherein a small voltage is applied across the STT-MRAM cell 10 and thecurrent through the STT-MRAM cell 10 is measured, where a large currentrepresents a “0”, representing a parallel orientation, and small currentrepresents a “1”.

Giant Spin Hall (GSH) Effect

The Giant Spin Hall effect is an efficient mechanism for generating spinpolarized currents. A charge current passing through a heavy metal layersuch as Ta, Pt or W have been experimentally demonstrated to generatein-plane spin polarized currents.

Recent advancements with the possibility of generating spin polarizedcurrent using charge current in heavy metals has led to the realizationof the Giant Spin Hall (GSH) effect based MRAM (also known asspin-orbit-torque MRAM; SOT-MRAM). Compared to STT-MRAMs, GSH-MRAMshowcase significant improvement in write energy along with thepossibility to independently co-optimize the read and write operationsdue to their decoupled read and write current paths. GSH effect alsoenables the possibility of achieving a differential storage due to thesimultaneous generation of opposite polarized spin currents. However,both the single ended and differential memory designs based on GSHeffect require multiple access transistors leading to a significant areapenalty. Also, the spin injection efficiency which is directlyproportional to the spin hall angle (θ_(SH)<0.3) is low for these heavymetals. This results in performance degradation and energy inefficiency.Another drawback with GSH-MRAMs is that they can only switch IMA magnetswithout the presence of any external magnetic field or geometricalchanges to the ferromagnet. As PMA magnets are known to be more energyefficient in switching and thermally stable than IMA, GSH-MRAMs offerlimited performance and energy benefits. Therefore, there arises a needto explore new memory technologies to harness the full potential ofspin-based storage.

Referring to FIG. 1 b , a schematic of GSH effect in heavy metal leadingto magnetization switching in an MTJ is shown, which provides athree-terminal device structure of the GSH effect-based spin device. TheGSH effect is mainly used for switching magnetization of in-planemagnetic anisotropy (IMA) magnets. Deterministic switching ofperpendicular magnetic anisotropy (PMA) magnets using GSH effectrequires externally assisted magnetic field to break the symmetry orcomplex design modifications to the MTJ geometry. The major advantagewith GSH effect-based magnetization switching is the low writecurrent/energy when compared to the STT-based magnetization switching.

The generated spin current (I_(S)) to charge current (I_(C)) ratio whichis also known as the spin injection efficiency is directly proportionalto the spin hall angle, θ_(SH). Experiments have shown θ_(SH) ˜0.1-0.3for heavy metals, resulting in low spin injection efficiency.Furthermore, the efficiency of GSH effect is impacted by the spin-fliplength (As), which characterizes the mean distance between spin-flippingcollisions. As has been calculated to be about 1-2 nm for heavy metalswith large GSH effect.

The three terminal device structure of the GSH effect-based spin deviceshown in FIG. 1 b , mitigates the read-write conflict of the twoterminal STT-MRAM due to the separation of read-write paths. Moreover,such an approach has shown to be promising for energy-efficient storagecompared to STT-MRAM. Several bit-cell designs have been proposed usingthe GSH effect, including a single ended GSH-MRAM, FIG. 1 c , whichshows a circuit schematic of GSH-MRAM which includes a read and writeaccess transistor for single-ended memory, and a differential DGSH-MRAM,FIG. 1 d , which shows a differential DGSH-MRAM bit-cell schematic whichincludes a read and write access transistor for each side of thedifferential DGSH-MRAM cell. Write operation is achieved by turning ONthe write access transistor and depending on the direction of chargecurrent flow (which determines the spin current polarization), the MTJstate is stored. The spin current interacting with the MTJ todeterministically switch the magnetization is calculated as:

$\begin{matrix}{I_{S} = {\frac{A_{MTJ}}{A_{HM}}*\theta_{SH}*I_{C}}} & (1)\end{matrix}$where A_(MTJ) and A_(HM) are the cross-sectional area of MTJ and heavymetal, respectively. The read operation is carried out by turning ON theread access transistor and sensing the resistance state of the MTJ(parallel (P) or anti-parallel (AP)). As the read and write paths aredecoupled, they can be optimized independently.

Utilizing the opposite spin generation at the top and bottom surfaces ofthe heavy metal, a differential GSH-MRAM (DGSH-MRAM) was proposed inwith two MTJs placed on either side of the heavy metal layer (see FIG. 1d ), representing a complimentary bit storage in the memory cell. Thewrite operation remains the same as GSH-MRAM while the read is achievedusing differential sensing, leading to higher sense margins. However,compared to GSH-MRAM, two more additional transistors are required toselectively access a bit cell in an array without disturbing theunassessed cells. Furthermore, fabrication of differential MTJs on thetop and bottom sides of the heavy metal increases processingcomplexities and costs.

The above mentioned GSH effect-based memory designs have been proposedto switch IMA based MTJs, since only in-plane spin polarized currentsare generated in the heavy metals. IMA magnets are not suitable forultra-scaled dimensions due the limit on the aspect ratio of the freelayer as well as low thermal stability. In comparison, PMA magnets aremore stable and robust at scaled dimensions with high packing density,which is mainly attributed to the absence of in-plane shape magneticanisotropy. Moreover, due to the absence of de-magnetization fields,lower energy is required for magnetization switching in PMA magnetscompared to IMA, even at iso-thermal stability. Although, GSHeffect-based PMA switching has been demonstrated with external magneticfield, or GSH assisted STT switching or a local di-polar field orintroducing tilted anisotropy in the ferromagnet, the feasibility ofachieving such a design change in scaled, high density technologies isyet to be explored. Moreover, the requirement of additional accesstransistors for GSH effect-based bit-cell designs leads to large areaoverheads which also increases the energy consumption for bit-line andword-line charging.

These issues are addressed by utilizing the valley-coupled-Spin Hall(VSH) effect in monolayer WSe₂ to design MRAMs based on PMA magnets. TheVSH effect is suited to switch PMA magnets, which promises higher energyefficiency in the memory cells of the present disclosure eliminating theneed for access transistors altogether.

Towards this end, the present disclosure presents novel memory cellsprovided in FIGS. 2 a and 2 b . With reference back to FIGS. 2 a and 2 b, a single-ended memory device and a differential memory device areshown, respectively. Single-ended VSH-MRAM includes one arm (alsoreferred to herein as a first wing) along which the transverse spincurrent flows. On the other hand, the differential DVSH-MRAM containstwo arms (also referred to herein as a first wing and a second wing) forcomplementary spin current flow. In the single ended design, a PMA MTJis integrated on top of the arm of the monolayer TMD spin generator asshown in the FIG. 2 a whose free layer (FL) is used for non-volatilemagnetic storage. In the differential design, as shown in FIG. 2 b , twoPMA MTJs storing true and complementary values are integrated on the twoarms of the spin generator. The read terminals of the memory devices(connected to the pinned layer of the read MTJs (see FIGS. 2 a and 2 b )are used to sense the bit-information stored. By virtue of VSH-basedwrite and MTJ-based read, the memory devices according to the presentdisclosure advantageously feature decoupled read-write paths.

The VSH effect in monolayer WSe₂ generates out-of-plane spin current(I_(S)), which interacts with the MTJ through spin torque to switch theFree Layer magnetization. Since VSH effect leads to the flow of oppositespin currents in divergent directions, the DVSH-MRAM according to thepresent disclosure is able to seamlessly store and switch both true andcomplementary bits. The direction of the charge current (I_(C))(controlled by the polarity of drain-to-source voltage (V_(DS)))determines the polarization of the spin current (I_(S+)/I_(S−)) flowingtowards the MTJ(s). Referring to FIGS. 2 c and 2 d which arecross-sectional representations of the VSH-MRAM and DVSH-MRAM accordingto the present disclosure, the out-of-plane spin currents are shown.When the current flows from the drain to source terminals, I_(S+) flowstowards the MTJ in VSH-MRAM and I_(S+)/I_(S−) flow towards theright/left MTJ (MTJ_(R/L)) in DVSH-MRAM. These spin currents generatespin torque leading to parallel (P) state in the MTJ of VSH-MRAM, and Pand anti-parallel (AP) states in MTJ_(R) and MTJ_(L) respectively, inDVSH-MRAM. Current is passed in the opposite direction to store theopposite states. This corresponds to the write operation of the memorydevices of the present disclosure. For reading the bit-information, theresistance difference between the P and AP states of MTJs are measured.The read current flows from the source and drain terminals of thetransistor to the read terminal of MTJs, in a ‘T’/‘H’ shape asillustrated in FIGS. 2 c and 2 d , for VSH/DVSH-MRAMs. Based on thecurrent sensed at the read terminals (which depend on the state of MTJ,P/AP), the bit-information stored is retrieved. It is important to notethat VSH-MRAM achieves single-ended sensing using a reference currentsource, while DVSH-MRAM enables differential sensing leading to highersense margins and self-referenced operation.

In order to read the MTJs, a current based reconfigurable senseamplifier which can dynamically switch its operation betweendifferential sensing mode (for memory-read) and single ended sensingmode is provided in FIGS. 2 e and 2 f which are circuit diagrams forthese two sense amplifier configuration (i.e., single-ended in FIG. 2 eand differential in FIG. 2 f ). The basic configuration is shown in FIG.2 e which is for VSH-MRAM (i.e., the single-ended version), acurrent-mirror based sense amplifier along with a reference currentgeneration circuit is used, as is known to a person having ordinaryskill in the art. During the standard memory-read mode, single endedsense amplifier (see FIG. 2 e ) to read the bits stored in VSH-MRAMs. Areference cell is used to generate a reference current which is theaverage of current for the parallel mode and current for theanti-parallel mode (i.e., I_(P)+I_(AP)/2), which is then compared withthe current sensed (I_(SL)) at the sense line. If I_(SL)=I_(P), then thesense amp output is ‘1’, else if I_(SL)=I_(AP), then sense amp output is‘0’. Similarly, for DVSH-MRAM, a differential sense amplifier is used(see FIG. 2 f ) to read the bit-information. Here, the sensed currentsare I_(SL) and I_(SLB) which compared with each other to determine thebit-stored. If I_(SL)=I_(P) and I_(SLB)=I_(AP), then the sense ampoutput is ‘1’, else it is ‘0’.

One unique aspect of the memory devices of the present disclosure is theintegrated back gate, which enables modulation of the I_(C), I_(S) andhence the switching characteristics of the PMA magnets (gatecontrollability quantified later). In the present disclosure, thisaspect is utilized for compact memory design, i.e., an −MTJ based memorywithout access transistor.

In order to better explain the memory cell of the present disclosure,first, the VSH effect is further described.

Valley-Coupled-Spin Hall (VSH) Effect

Monolayer transition metal dichalcogenides (TMDs) are multi-valley 2Dsemiconductors (FIG. 3 a , which shows a schematic of a band structureof WSe₂ showcasing spin-valley coupling resulting in VSH effect) withinherent broken inversion and preserved time reversal symmetries. Timereversal symmetry requires that the spin polarization in the K valleyand K′ valley must be opposite (illustrated as solid and hollow arrowsin FIG. 3 a ), which in combination with the large spin splitting(Δ_(SP)) in the valence band for TMDs such as WSe₂, gives rise to holesin the K and K′ valley with opposite signs of spin polarization at theFermi level. As a result, carriers in the K valley and K′ valley of thevalence band (p-type) possess nonzero Berry curvature (Ω) such thatΩ(K)=−Ω(K′). The resultant transverse carrier velocity leads tovalley-coupled spin currents on the application of electric field. Thisphenomenon is called the VSH effect. VSH effect in WSe₂ generatesout-of-plane spin polarized currents (I_(S+) and I_(S−) shown inlong-dashed and dotted arrows, respective; see FIG. 3 a ) which canswitch PMA magnets without any external magnetic field (BExT) or complexchanges to the MTJ structure, unlike GSH-effect based memory devices.

It has been demonstrated that monolayer TMDs exhibit a large valley-hallangle, θ_(VH)˜1 at 25° C. Due to the existence of strong spin-valleycoupling in monolayer WSe₂ (as a result of large Δ_(SP)), the θ_(SH) isexpected to be equal to θ_(VH), i.e., θ_(SH)˜1. The large θ_(SH)corresponds to high spin injection efficiency which can potentially leadto enhanced energy efficiency during magnetization switching. Incontrast, GSH effect exhibit relatively much smaller θ_(SH)˜0.1-0.3.Moreover, VSH effect resulting in out-of-plane spin generation exhibitsλ_(S) of 0.5-1 μm (unlike GSH effect in heavy metals; λ_(S)˜1-2 nm). Thelarge θ_(SH) and λ_(S) in monolayer WSe₂ advantageously and surprisinglyprovide novel opportunities for memory applications.

Utilizing the unique attributes of VSH effect in conjunction with spintorque physics (as shown in FIGS. 3 b and 3 c which are schematics ofSTT for switching PMA magnet and the novel idea of coupling VSH effectand spin torque for NVM design, respectively and according to thepresent disclosure), a valley-coupled spintronic devices is disclosedwith a small footprint for energy-efficient data storage.

Towards this end, the present disclosure provides an energy-efficientVSH effect based single-ended and differential spintronic memory devicesand their access-transistor-less arrays (i.e., memory cells withoutaccess transistors). The single-ended and differential design arereferred herein to as VSH- and DVSH-MRAMs, respectively. Detailed arrayand system-level analysis are also provided for the VSH-MRAMs andDVSH-MRAMs of the present disclosure in comparison with existingGSH-MRAMs and DGSH-MRAMs in the context of a general-purpose processorand an intermittently-powered system.

Two types of measurements were performed, as illustrated in FIGS. 4 a, 4b, and 5 a-5 d . A conventional four-probe measurement (FIG. 4 a , is aschematic of a four-probe measurement scheme) was conducted to extractsheet resistance (ρ), contact resistance (R_(C)) and total resistance(R_(TOT)). Results from these measurement are provided in FIG. 4 b whichis a graph of resistance vs. V_(GS) showing the three aforementionedtypes of resistance, i.e., ρ, R_(C), and R_(TOT) vs. V_(GS). Thenon-local (NL) measurements were performed to probe the Hall voltageinduced by any carrier distributions due to the VSH and its reciprocaleffect (see FIG. 5 a which is a schematic of a non-local measurementsetup). Note that only the ON state of the WSe₂ device is considered forvalley-coupled-spin transport in the discussions below, since access toholes in the valence band is necessary. Referring to FIGS. 5 b and 5 c ,graphs of gate control of charge current (I_(C)) and NL resistance(R_(NL)=V_(NL)/I_(C)) for different device samples with arm lengths(L_(A)) equal to 2 μm, 3 μm and 5 μm are shown. R_(NL) vs L_(A) atV_(GS)=−60 V was used to extract the spin flip length, λ_(S)=550 nm fromthe fitting of R_(NL)∝e (−L_(A)/λ_(S)). The results are shown in FIG. 5d which is a graph of R_(NL) (R_(NL)=V_(NL)/I_(C)) vs. arm length(L_(A)) of the memory device. It is appreciated that the spin-generatoris a p-type device and therefore it requires negative gate-to-sourcevoltages to turn it ON.

The memory device of the present disclosure was simulated using aself-consistent simulation framework in SPICE for the valley-coupledspintronic memory device/array of the present disclosure. Referring toFIG. 6 a , a flowchart for a self-consistent simulation framework isshown for the memory device of the present disclosure. Monolayer WSe₂electrostatics is modeled using the capacitance network model known inthe art with modification for back-gated device used in the presentdisclosure. Further, the charge current is modeled using the continuityequations for drift-diffusion transport (see calibration in FIG. 6 bwhich is a calibration graph of I_(GS) vs. V_(GS) for experimental vs.simulation runs). The charge current is then used in conjunction withthe Valley Spin Hall effect model, which calculates the spin currentbased on the experimental θ_(SH) and λ_(S) values. I_(S) interactingwith the free layer of MTJ is calculated as:

$\begin{matrix}{I_{S} = {\frac{D_{MTJ}}{L_{G}}*\theta_{SH}*I_{C}}} & (2)\end{matrix}$where D_(MTJ) is the diameter of MTJ (circular) and L_(G) is gate lengthof the transistor. Referring to FIG. 2 a-2 d two different embodiments(FIGS. 2 a,2 c and 2 b,2 d ) of the memory cell according to the presentdisclosure are provided. Spin diffusion and interface scattering areconsidered in the monolayer WSe₂ channel while calculating the spincurrent flow as for the method disclosed herein.Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation is used to model theswitching dynamics of the PMA magnet, which serves as the free layer (FLor Free Layer) of a magnetic tunnel junction (MTJ) formed on top of theTMD (as described later). For sensing, the MTJ resistance (R_(MTJ))model is obtained from the prior art, known to a person having ordinaryskill in the art. Further, as discussed below, the read path is ‘T’/‘H’shaped. To properly account for the sensed currents, a distributedresistive network is used (as shown in FIG. 6 c which is a schematic ofa distributed resistance network for sensing MTJs) based on theconductance of WSe₂ layer and the shape of the read path. Therefore, theread path includes the resistance of the MTJ as well as that ofconducting WSe₂ layer. The sensed currents are used to read thebit-information stored and also perform computation in memory. Contactresistances at the drain terminal is included, source terminal andMTJ-TMD interface based on (accounting for Schottky barrier). Referringto FIG. 6 d , the simulations parameters used in the present disclosureare listed. A minimum gate length (L_(G)) of 45 nm is used in thesimulations.

The process flow for the fabrication of the device of the presentdisclosure is now disclosed. Chemical vapor deposition (CVD) grown WSe₂films were transferred to 90 nm SiO₂ substrates with highly dopedsilicon on the back side. Doped Si serves as the integrated back gatefor controlling the flow of IC and IS (as explained later). Standarde-beam lithography using PMMA A4 950 resist was employed to patternelectric contacts on the CVD WSe₂ flakes. Ti/Pd (0.5/50 nm) wasdeposited in an e-beam evaporator followed by a lift-off process inacetone. CVD grown BN film was transferred from Cu foil onto the devicesthrough a process that involves etching the Cu foil with iron chloride(FeCl₃) and immersing it in diluted HCl and DI water alternatingly forfew times before scooping up. This BN layer was inserted to minimizedevice degradation from PMMA residues after the RIE etching process, butnot necessary for general (D)VSH-MRAM fabrication. RIE etching mask wasdefined by e-beam lithography using PMMA A4 950 resist and BN/WSe₂flakes were etched using Ar/SF6 for 10 seconds. The final devicesunderwent nitric oxide (NO) furnace annealing at 150° C. for two hoursfollowed by vacuum annealing (˜10-8 torr) at 250° C. for four hours tominimize PMMA residue and threshold voltage shift due to trap charges.

As discussed above, charge current flowing through the monolayer WSe₂generates transverse spin currents. Referring to FIG. 7 a , a graph ofsimulated gate voltage (V_(GS)) vs. modulated charge and spin currentflow are shown. The polarity of V_(DS) determines the polarization forthe spin current flowing towards the MTJ resulting in correspondingmagnetization switching as illustrated in FIG. 7 b which is a graph ofmagnetization vs. time for V_(DS)=1 V and V_(DS)=−1 V. Since, the gatevoltage controls the carrier density in the WSe₂ layer, themagnetization switching time is gate controllable as shown in FIG. 7 cwhich is a graph of switching time vs. V_(GS). Higher |V_(GS)|corresponds to larger I_(C) (or I_(S)) which in turn results in smallermagnetization switching time. For the VSH-MRAM or DVSH-MRAM cellsaccording to the present disclosure, switching time ranging from 3.2 nsto 1.5 ns for V_(GS)=−1.0 V to −1.2 V are achieved. Note, themagnetization switching time for VSH and DVSH-MRAMs devices remainsimilar because of the inherent and concurrent generation of thecomplementary spin currents (I_(S+) and I_(S)) due to the VSH effect. Incomparison, prior art GSH-MRAM cells (see, e.g., FIG. 1 b ) exhibit aswitching time ranging from 11.6 ns to 4.0 ns for V_(DD)=1.0 V to 1.2 V.The benefits are attributed to the easier switching of PMA magnets inVSH-MRAMs when compared to IMA in GSH-MRAMs, mainly due to lowerswitching current requirement for a given thermal stability.

It should be noted that when V_(GS)=0 V, the VSH memory device is OFFand the magnetization state is retained due to the non-volatility of theferromagnet. To read or change the magnetization state stored, thedevice has to be turned ON (negative V_(GS), since the inherenttransistor is equivalent to a P-Channel device). As discussed furtherbelow, during read, even though the device is ON, no charge currentflows from the drain to source (to avoid generation of spin current dueto VSH effect), thereby safeguarding the magnetization state from anyVSH-induced disturbance.

In order to use the memory cells of the present disclosure in a memoryarray reference is now made to FIGS. 8 a-8 b . FIG. 8 a is a schematicof a memory array architecture for VSH-MRAM while FIG. 8 b is aschematic of a memory array architecture for DVSH-MRAM. In both cases,these arrays feature access-transistor-less cells by virtue of theintegrated gate of the memory devices of the present disclosure. Theintegrated gates of all the memory cells in the same row are connectedto the word-line (WL). The source, drain and the read-ports of all thecells in the same column are connected to bit-line (BL), bit-line-bar(BLB) and sense-line/sense-line-bar (SL/SLB) respectively. Theintegrated back gate provides selective word access in the array asdiscussed later. Read, write, and hold operations are described inreference to Table 1. Each of these modes are discussed below.

TABLE 1 Operation bias conditions for (D)VSH-MRAMs Underlined:Precharged WL BL BLB SL SLB WRITE 0 V_(DD)/0 0/V_(DD) V_(DD) V_(DD) READ0 V_(DD) V_(DD) V_(DD)-V_(READ) V_(DD)-V_(READ) HOLD V_(DD) V_(DD)V_(DD) V_(DD) V_(DD)

(i) Write: For writing into the memory cell of the present disclosure, 0V is applied to WL of the accessed word (it should be noted again thatthe memory devices are p-type). BLs and BLBs are then asserted accordingto the bit-information which is to be stored (as discussed above,direction of charge current determines the bit stored). SLs and SLBs arekept pre-charged (and floating) at V_(DD) (1.0 V). This creates a highimpendence path for the charge current to flow through MTJ, avoidingaccidental magnetization switching due to STT effect. Now, supposebit-‘0’ is written. 0V/V_(DD) is applied to BL/BLB in both VSH- andDVSH-MRAMs (e.g., V_(DD)=1.1 V). VSH effect as discussed above, flipsthe Free Layer of MTJ in VSH-MRAM to positive magnetization state(Mz=+1) and the MTJ comes to the Parallel (P) configuration. While forDVSH-MRAM, Free Layers of MTJ_(R) and MTJ_(L) flip to positive andnegative magnetization states (M_(Z)=+1 and −1) which brings them to Pand anti-parallel (AP) configurations respectively, corresponding tobit-‘0’. On the other hand, for writing bit-‘1’, V_(DD)/0V is applied toBL and BLB, and the VSH effect leads to storage of M_(Z)=−1 in FL of MTJ(AP) of VSH-MRAM and M_(Z)=−1/+1 in FL of MTJ_(R)(AP)/MTJ_(L)(P) ofDVSH-MRAM. Note, in DVSH-MRAM, the true bit value is stored in MTJ_(R)while the complementary bit is stored in MTJ_(L). After write, all linesare pre-charged to V_(DD). Note, the BLs/BLBs, SLs/SLBs of theunaccessed cells are precharged to V_(DD), while the WLs are driven toV_(DD) to avoid any unintentional M_(Z) switching. This corresponds toV_(GS)=V_(DS)=0V in the unaccessed memory devices resulting ininsignificant charge/spin current flow (no write disturbance).

(ii) Read: For reading the bit-information, 0V is applied to WL andV_(DD) to BLs and BLBs of the accessed word. The SLs and SLBs are drivento V_(DD)-V_(READ). This brings the memory devices of the accessed wordto the ON state and there exists a read current flow between the senseline(s) and source/drain terminals of the memory cell (due to thevoltage difference, V_(READ)=0.4 V). The read current (I_(SL)/I_(SLB))depends on the resistance of the MTJ storing P or AP configuration (asdiscussed above). For VSH-MRAM, I_(P) is the current sensed at SL whenthe memory cell stores bit-‘0’ (parallel configuration of MTJ) andI_(AP) is the current sensed when bit-‘1’ is stored (anti-parallel MTJ),where I_(P)>I_(AP). For DVSH-MRAM, I_(P) (I_(AP)) and I_(AP) (I_(P)) arethe currents sensed at SL and SLB when the bit stored is ‘0’ (‘1’).VSH-MRAMs employs single-ended sensing, where a reference cell current,I_(REF)=(I_(P)+I_(AP))/2 is used to compare the current flowing throughSL (I_(SL)). On the other hand, DVSH-MRAM is self-referenced. After theread operation, all lines are pre-charged to V_(DD). Note, similar tothe write operation, the BLs/BLBs and SLs/SLBs of the unaccessed cellsare precharged to V_(DD) and the WLs are driven to V_(DD) to avoid anydisturbances.

(iii) Hold/Sleep: During the hold operation, all the lines of the memoryarray are precharged to V_(DD). This process also ensures minimal energyconsumption during charging/dis-charging of bit-lines for memory'sread/write operations. On the other hand, during the sleep mode, i.e.,when the power supply is completely shut down for a long time, all lines(BL/BLB, SL/SLB and WL) are driven to 0V. In both these cases (hold andsleep modes), the non-volatility of the magnetization in FL of MTJensures storage of the bit-information even in the absence of anyexternal power supply leading to zero stand-by leakage power.

To evaluate the performance of the memory devices of the presentdisclosure, a comparison of these memory cells is made with existingGSH/DGSH-MRAMs for a 1 MB array (8 banks, each bank with 1024 rows and1024 columns) with 32-bit words and evaluate the area, write and readmetrics. Iso-energy barrier of about 55 K_(B)T (>10 years of retention)for PMA MTJs in the VSH/DVSH-MRAMs of the present disclosure and IMAMTJs in GSH/DGSH-MRAMs is considered for a fair evaluation. This isachieved by tuning the device geometry. Referring to FIG. 9 a bar graphis shown which illustrates the array level comparison of these memorydesigns.

First, the VSH/DVSH-MRAMs of the present disclosure achieve 66/62% lowerbit-cell area compared to GSH/DGSH-MRAMs. This is attributed to theaccess transistor less array design achieved due to the uniqueintegrated back gate feature. The lower bit-cell area leads to reducedmetal-line capacitances (for word-lines/bit-lines) in the memory array.This feature, along with other properties of the VSH effect, enhancesthe energy efficiencies for memory operations for VSH/DVSH-MRAM.

The write metrics of the proposed VSH and DVSH-MRAMs remain similarbecause of the inherent and concurrent generation of I_(S+) and I_(S−)due to the VSH effect (as discussed above). However, the same propertydoes not hold true for the GSH and DGSH-MRAMs because of differentnumber of access transistors (one and two respectively) driving thewrite operation. The analysis shows that VSH/DVSH-MRAMs achieve 59%/67%lower write energy (WE) and 50%/11% lower write time (WT) compared tothe GSH/DGSH-MRAM. This is attributed to two factors. First, the uniquegeneration of out-of-plane spin currents with VSH-effect enables theswitching of PMA magnets, unlike GSH effect which can only switch IMAmagnets. It is understood that IMA switching is relatively lessenergy-efficient than PMA switching due to demagnetization fields.Second, lower cell area in the proposed memories results in reduced timeand energy consumption for bit-line charging/dis-charging during thewrite operation.

The PMA MTJs in VSH/DVSH-MRAMs exhibit higher resistance due to itssmaller area compared to IMA MTJs in GSH/DGSH-MRAMs at iso-energybarrier. Moreover, the WSe₂ FET is more resistive than a silicon-basedFET used in (D)GSH-MRAMs due to lower mobility. This results in lowersensing currents in VSH/DVSH-MRAMs during the read operation. At thesame time, lower area of the proposed memory array due to the integratedback gate feature reduces the bit-line charging/dis-charging energy.Both these factors lead to 74%-77% lower read energy consumption in theproposed memories. However, the lower sensed currents result in 45%lower sense margin for VSH/DVSH-MRAMs compared to GSH/DGSH-MRAMs, atV_(READ)=0.4 V. At iso-sense margin (achieved by reducing V_(READ) for(D)GSH-MRAM to 0.15 V), 35%/41% lower read energy is achieved byVSH/DVSH-MRAMs.

With respect to the single-ended VSH-MRAMs, differential DVSH-MRAMsexhibit 50% improved sense margin with a penalty of 64% increase in readenergy, attributed to the additional sense-line (SLB) charging energy.However, at iso-sense margin, achieved by reducing V_(READ) of DVSH-MRAMto 0.2 V, similar read energies are observed for VSH and DVSH-MRAMs. Toevaluate the system-level benefits of the VSH-MRAM and DVSH-MRAM designsof the present disclosure, performance is measured when used as an L2cache (unified memory) in a general-purpose processor.

Referring to FIG. 10 , a bar graph of normalized memory energy forvarious SPEC benchmarks for DGSH, GSH, DVSH and VSH-MRAMs is provided.It also shows the energy consumed by the major L2 cache operations,which are, reads during L2-hits, reads and writes duringL2-replacements, and writes during L2-misses and L2-hits. Across a suiteof SPEC2K6 benchmarks, VSH-MRAM and DVSH-MRAM exhibit similar L2 cacheenergy due to similar write and read energies (at iso-sense margin) asdiscussed above. In comparison with DGSH-MRAM and GSH-MRAM, the proposedVSH-MRAMs and DVSH-MRAMs show 2.63-3.14× and 2.19-2.50× reduction in thetotal L2 cache energy, respectively. Further, the applications (e.g.,milc) with a lower read/write ratio show higher benefits. This isbecause the memory cell designs of the present disclosure can performwrites far more efficiently compared to the baseline designs.

Due to the tight energy constraints of intermittently powered systems,the more energy-efficient design for GSH memory is chosen for thisanalysis (single ended GSH-MRAM consumes less energy than thedifferential design—see FIG. 9 ). Also, for fair comparison, theanalysis covers only VSH-MRAM (although both VSH and DVSH MRAMs showsimilar energy efficiency at iso-sense margin as discussed before).Referring to FIG. 11 , a schematic of a simulation framework is shownwhich is used, according to the present disclosure. The system-levelsimulations are based on the TI MSP430FR5739 microcontroller-based edgeplatform running at 24 MHz and use a unified 32 kB NVM based on theproposed VSH-MRAM (with iso-sense margin of 1.85 μA compared to thebaseline GSH-MRAM). The system is powered using an energy harvestingsource that charges a supply capacitor of 10 nF. The systemconfiguration and set of real benchmarks used are shown in Tables 2 and3. Results are showcased in FIGS. 12 a and 12 b which are bar graphsrepresenting normalized system energy consumption of VSH and GSH-MRAMfor (a) synthetic and (b) real application benchmarks, respectively,which depict total memory energy consumption for iso-work conditions.Note, the energy numbers in FIGS. 12 a and 12 b are normalized toGSH-MRAM energy consumption.

TABLE 2 system configurations for the simulation framework FeatureDescription Microcontroller Architecture 16-bit RISC-based TIMSP430FR5739 Total #Config Registers 165 Memory Architecture (Capacity)Unified NVM (32 KB) Proc. Pipeline Stages Single Cycle (No Pipeline)Frequency of operation 24 MHz Supply Capacitance 10 nF

TABLE 3 Application benchmark used for evaluation Benchmark Description(C) AEC Perform Advanced Encryption Standard-based encryption on 256messages CRC Compute 16-bit Cyclic Redundancy Code for error-correctionof 256 messages FFT Execute Fast Fourier Transform on sample data.MAT-MUL Compute Matrix Multiplication among two matrices. RSA RunRivest-Shamir-Adleman cryptography Description SENSE Sample 100 Sensorreadings and perform various statistical computation

The energy savings obtained from using VSH-MRAMs compared to GSH-MRAMsdepend primarily on the program characteristics, i.e., total number ofreads and writes during program execution while executing a specificapplication. A set of synthetic benchmarks were constructed where thefraction of total memory read and write instructions are varied with aconstant checkpoint size of 128B and total number of instructions(100K). Here, the expression {rd:0.25, wr:0.25} represents that 25% ofthe total instructions are memory reads, 25% are memory writes, and therest are normal computational operations. With reference back to FIG. 12a , it can be observed that the VSH-MRAMs of the present disclosureachieve significant energy benefits over GSH-MRAMs, ranging from 35% to59% for a wide spectrum of synthetic memory instructions. This isattributed to the improved read-write energy (at iso-sense margins). Forreal application benchmarks, VHS-MRAMs exhibit energy savings in therange of 40%-49% (1.66×-1.98×) and 45% (1.80×) on an average compared toGSH-MRAMs (see FIG. 12 b ).

Those having ordinary skill in the art will recognize that numerousmodifications can be made to the specific implementations describedabove. The implementations should not be limited to the particularlimitations described. Other implementations may be possible.

The invention claimed is:
 1. A memory cell, comprising: a semiconductorlayer; a first electrode coupled to the semiconductor layer; a secondelectrode coupled to the semiconductor layer, wherein the first andsecond electrodes are separated from one another along a first axis andwherein the semiconductor layer extends beyond the first axis along asecond axis substantially perpendicular to the first axis, therebyforming a first wing; a third electrode separated from the semiconductorlayer by an insulating layer; a first magnetic tunnel junction (MTJ)disposed on the first wing; and a first read electrode coupled to thefirst MTJ.
 2. The memory cell of claim 1, wherein placing an on-statevoltage on the third electrode causes the semiconductor layer to switchon allowing conduction of electrical current between the first and thesecond electrodes establishing a charge current which generates a firstspin current perpendicular to the charge current, which in turngenerates a first magnetic spin which then generates spin orbit torqueresulting in flipping of the first MTJ thereby causing a write operationto the first MTJ.
 3. The memory cell of claim 2, wherein the on-statevoltage represents a voltage drop above a voltage threshold across thefirst and third electrodes.
 4. The memory cell of claim 3, wherein i)placing the on-state voltage on the third electrode causes thesemiconductor layer to switch on, and ii) placing a first read voltageacross the first electrode and the first read electrode and placingsubstantially the first read voltage across the second electrode and thefirst read electrode thereby generating a first read current, whereinthe first read current represents a value corresponding to state of thefirst MTJ, thereby performing a read operation from the first MTJ,wherein an off-state voltage represents a voltage drop below the voltagethreshold across the first and third electrodes.
 5. The memory cell ofclaim 1, wherein the semiconductor layer includes WSe₂, MoS₂, MoSe₂,WS₂, or a combination thereof.
 6. The memory cell of claim 5, whereinthe semiconductor layer further extends beyond the first and secondelectrodes forming a second wing, opposite the first wing along thesecond axis.
 7. The memory cell of claim 6, further comprising: a secondMTJ disposed on the second wing, the second MTJ having a structureessentially identical to the first MTJ; and a second read electrodecoupled to the second MTJ.
 8. The memory cell of claim 7, whereinplacement of an on-state voltage on the third electrode furthergenerates a second spin current opposite the first spin currentperpendicular to the charge current, which in turn generates a secondmagnetic spin opposite the first magnetic spin which then generates spinorbit torque resulting in flipping of the second MTJ opposite the firstMTJ thereby causing a write operation to the second MTJ.
 9. The memorycell of claim 8, wherein i) placing the on-state voltage on the thirdelectrode, and ii) placing a second read voltage across the firstelectrode and the second read electrode and placing substantially thesecond read voltage across the second electrode and the second readelectrode thereby generating a second read current, wherein the secondread current represents a value corresponding to the state of the firstMTJ, thereby performing a read operation from the second MTJ, wherein anoff-state voltage represents a voltage drop below the voltage thresholdacross the first and third electrodes.
 10. An array of memory cells,comprising: a plurality of memory cells dispersed in one or more rowsand one or more columns, each comprising: a first electrode coupled tothe semiconductor layer, a second electrode coupled to the semiconductorlayer, wherein the first and second electrodes are separated from oneanother along a first axis and wherein the semiconductor layer extendsbeyond the first axis along a second axis substantially perpendicular tothe first axis, thereby forming a first wing, a third electrodeseparated from the semiconductor layer by an insulating layer; a firstmagnetic tunnel junction (MTJ) disposed on the first wing, and a firstread electrode coupled to the first MTJ; a wordline (WL) for each row ofthe one or more rows coupled to the third electrodes of each of theassociated memory cells in the associated row; a bit line (BL) for eachcolumn of the one or more columns coupled to the first electrodes ofeach of the associated memory cells in the associated column; a bit linebar (BLB) for each column of the one or more columns coupled to thesecond electrodes of each of the associated memory cells in theassociated column; and a select line (SL) for each column of the one ormore columns coupled to the first read electrodes of each of theassociated memory cells in the associated column, wherein by selectivelyactivating the WL, BL, BLB, and SL, each of the memory cells is accessedfor writing or reading i) one memory cell at a time, ii) a row of memorycells at a time, iii) column of memory cells at a time, or iv) the arrayat a time.
 11. The array of memory cells of claim 10, wherein placing anon-state voltage on the WL causes the semiconductor layer of theassociated memory cell to switch on allowing conduction of electricalcurrent between the first and the second electrodes establishing acharge current which generates a first spin current perpendicular to thecharge current, which in turn generates a first magnetic spin which thengenerates spin orbit torque resulting in flipping of the first MTJthereby causing a write operation to the first MTJ of the associatedmemory cell.
 12. The array of memory cells of claim 11, wherein theon-state voltage represents a voltage drop above a voltage thresholdacross the first and third electrodes of the associated memory cell. 13.The array of memory cells of claim 12, wherein i) placing the on-statevoltage on the third electrode causes the semiconductor layer to switchon, and ii) placing a first read voltage across the first electrode andthe first read electrode and placing substantially the first readvoltage across the second electrode and the first read electrode therebygenerating a first read current, wherein the first read currentrepresents a value corresponding to state of the first MTJ, therebyperforming a read operation from the first MTJ of the associated memorycell.
 14. The array of memory cells of claim 13, wherein an off-statevoltage represents a voltage drop below the voltage threshold across thefirst and third electrodes of the associated memory cell.
 15. The arrayof memory cells of claim 10, wherein the semiconductor layer includesWSe₂, MoS₂, MoSe₂, WS₂, or a combination thereof.
 16. The array ofmemory cells of claim 15, wherein the semiconductor layer of theassociated memory cell further extends beyond the first and secondelectrodes forming a second wing, opposite the first wing along thesecond axis.
 17. The array of memory cells of claim 16, furthercomprising: a second MTJ disposed on the second wing, the second MTJhaving a structure essentially identical to the first MTJ; and a secondread electrode coupled to the second MTJ.
 18. The array of memory cellsof claim 17, wherein placement of an on-state voltage on the thirdelectrode of the associated memory cell further generates a second spincurrent opposite the first spin current perpendicular to the chargecurrent, which in turn generates a second magnetic spin opposite thefirst magnetic spin which then generates spin orbit torque resulting inflipping of the second MTJ opposite the the first MTJ thereby causing awrite operation to the second MTJ.
 19. The array of memory cells ofclaim 18, wherein i) placing the on-state voltage on the thirdelectrode, and ii) placing a second read voltage across the firstelectrode and the second read electrode and placing substantially thesecond read voltage across the second electrode and the second readelectrode thereby generating a second read current, wherein the secondread current represents a value corresponding to the state of the firstMTJ, thereby performing a read operation from the second MTJ, wherein anoff-state voltage represents a voltage drop below the voltage thresholdacross the first and third electrodes of the associated memory cell.